This invention relates to an improved body for a dump truck, and in particular to a gate-free truck body for use in large-scale mining operations and a style of body that may be adapted for use on numerous types of mining truck chassis, after accounting for weight distribution, wheel base, gross vehicle weight, and other such factors.
Referring to FIGS. 1A and 1B, a typical large mining truck 10 has a chassis 12 capable of mounting a variety of bodies 14 having different shapes and designs. For various reasons, including the fact that different materials to be hauled have different densities, truck bodies may have different shapes, may be made of different materials, and may have different front wall 16, side walls 18a and 18b, floor 20, and canopy 22 designs. For example, some bodies are designed to haul low density materials, such as coal. Other bodies are designed to haul higher density materials including rock or overburden (the rock and dirt found above an ore or coal seam).
In general, the density of coal is about half that of overburden. Thus, a given volume of coal weighs approximately half what the same volume of overburden weighs, and so the volume of a truck body 14 designed to haul coal may be twice as large as a body designed to haul overburden without exceeding the carrying capacity of the truck chassis 12. However, having one set of trucks with bodies for coal and a second set of trucks with bodies for overburden can significantly increase mining costs because of the need to maintain two fleets of vehicles.
To avoid maintaining two sets of trucks, truck bodies 14 designed to perform the task of hauling both coal and overburden have been designed. These bodies are typically called multi-purpose bodies, or “combo” (short for combination) bodies. As a practical matter, because of the different densities of the hauled material, and because of the maximum load weight for which a body may be designed, the combination bodies may haul very different volumes of the different materials.
That is, when hauling low density coal the body may be loaded to a much higher height than is the case when hauling a high density material such as overburden. This difference is shown in FIGS. 2 and 3, which depict the heap height of a full capacity coal load using dark dashed lines 26, and the heap height of a full capacity overburden load using dark dotted lines 28, in two different styles of body 14. These figures also depict the side wall 18a and 18b heights of a full capacity coal load using light dashed lines 26a and the side wall 18a and 18b heights of a full capacity overburden load using light dotted lines 28a. As is evident in either design, while hauling overburden material the truck body typically provides significant reserve volume. This difference is required because hauling the same volume of overburden as coal will likely exceed the maximum carrying capacity of the truck 10.
In the last several years, significant efforts to analyze the characterization of payloads and prediction of dumping performance for various floor profiles have provided new insights into designs for combination bodies, and in particular adjustments of the angles of the floor of the combination body. For example, as shown in FIG. 2, one contemplated concept was for a “12+10” or 12/22 floor design, that is, a truck body floor having a front portion at a 12 degree angle to the horizontal, and a rear portion with an additional 10 degrees (thus a total of 22 degrees) angle to the horizontal. However, analysis of that concept indicated the design presented risks in shedding performance, that is, how the load dumps out of the body as the body is rotated up for unloading.
Shedding performance can be critical to the performance of a truck body. For overburden placed in a combination body, the payload may occupy approximately half of the body capacity. Thus, when placed in a position for proper axle distribution during road transport, a large portion of the rear of the body is empty. During the dump cycle, however, the material sheds onto the rear section or rear panel of the body, and may form a secondary heap on the rear panel, leading to unacceptable and potentially dangerous conditions especially when dumping over a ridge.
That is, typically, large dump trucks have two front tires 36 on the front axle, and four back tires 38 on the rear axle 39, and so the optimum load-carrying design places approximately ⅔ of the load on the four rear tires and ⅓ of the load on the two front tires. Depending upon the length of the inclined rear panel, the load can form a secondary heap on the back of the body 14 when dumping that then spans between the side walls 18a and 18b. Thus, as the load sheds off the body, the weight distribution (center of mass) of the load may shift aft, resulting in too much of the weight being carried by the back of the truck.
Because the fulcrum of the body being tipped is typically behind the rear tires 38 and axle 39, if at any time during dumping of a load the center of gravity of the load shifts too far to the rear, the front end of truck 10 can be tipped up, meaning that the cab 42 of the truck rises, sometimes several feet. Even if the weight distribution is not so skewed as to cause the cab to rise, if the center of mass of the load shifts too far, the lift cylinders 40 pushing the front of the body 14 up to dump the load may suddenly go from being under compression (pushing the body upward) to being under tension (the body tugging on the lift cylinders), something that can seriously damage the lift cylinders. Thus, improper weight distribution of the load during dumping can suddenly thrust the cab upward, create tension on the lift cylinders, and also cause a sudden dumping of a large portion of the load, which then shifts the center of gravity of the load forward again, thereby causing the elevated cab to drop and forcing the lift cylinders back into compression, further damaging the cylinders and perhaps frightening or even harming the truck driver. Furthermore, moving the pivot point to be in front of the rear tires may cause the body to strike the tires or the ground when dumping, and doing so will also reduce the leverage available to the lift cylinders, increasing the weight those cylinders must lift to dump the load.
As a result, it is important to maintain a proper front to back weight distribution of the load during dumping. Because analysis of the 12/22 body indicated there may be problems with load distribution when dumping from some trucks, a “12+5” or 12/17 dual slope floor combination haul body was designed for those types of trucks with a front portion of the floor angle at 12 degrees from horizontal with an additional 5 degrees incline for the rear portion of the floor resulting in a combined 17 degrees off horizontal. This design provided a relatively large volume or cavity for carrying material without making the body so long as to obstruct dumping as a result of, for example, the end of the body hitting the rear tires or the ground or previously dumped material. However, analysis indicated that, for several reasons the 12/17 body would not work on some chassis.
Because the rear portion of the load or heap is generally conical, for dense material loads there is often several feet between the where the heap strikes the sidewalls 18a and 18b and the rear of the floor 20. In a 12/22 body, the rear floor panel 44 is at an angle 10 degrees greater than the forward floor panel 46 and based on sliding friction will shed payload 10 degrees later than the forward floor. Assuming a 45 degree shed line (that is, assuming the material will shed from the heap when the angle of the surface of the material is at 45 degrees) and an initial heap sloped at 2:1 (that is, the heap will have an initial slope of about 26.5 degrees from the horizontal), individual layers of the heap will begin sliding at the point where the truck body 14 has rotated approximately 18.5 degrees, because (18.5 rotation)+(26.5 heap slope)=45 degrees. At this angle of body rotation, the rear floor panel, which was originally 12+10=22 degrees from the horizontal, will be at an angle of −3.5 degrees from horizontal (22 degrees minus the 18.5 degrees of rotation).
If the static coefficient of friction between the material and the floor is 0.61, the body 14 must rotate an additional 35 degrees for all of the material to shed freely off the rear of the floor 20. The forward floor panel 46 will begin to apply a thrust loading to the accumulating material on the rear panel 44 until the frictional resisting force is overcome, at which time the entire heap will slide as a unit. FIGS. 4 and 5 depict the difference in the thrust loading or the payload reaction as the body is dumping, including the load stress presented by the load on two different floors. FIG. 4 shows a 12/22 floor and FIG. 5 shows a 12/17 floor. As indicated in those figures, at 35 degrees of rotation, the 12/22 floor presents significantly more stress along the floor panels than the 12/17 floor, indicating a higher level of pressure. However, both these floors show the potential for secondary heaps to form on the rear panels of the bodies, as indicated by the darker sections on the rear panels shown in FIGS. 4 and 5.
A variety of combination bodies have been developed using similar floor designs. To date, many gate-free haul bodies designed to be mounted on chassis made by certain manufacturers are of the dual slope design and generally begin with a floor at an angle ranging from an initial slope of between 7 and 12 degrees from horizontal and then the remaining floor increases in angle near the pivot bore. This extra floor “kick” eliminates the need for a tailgate by increasing the length of the body while still maintaining an adequate volume or cavity for the payload.
Canopy loading is often required to attain the desired payload volume for coal while maintaining acceptable axle (tire) weight-bearing distributions. Even with these duel angle floor designs, the center of mass of the load may shift suddenly aft, resulting in tipping of the cab, or tension on the lift cylinders, or both. The success of these other designs is dependent on numerous factors, including the wheel base of the truck, the maximum load rating of the truck, the length of the body, stress on the body floor due to movement of a large mass of the load to the rear of the body without shedding, shedding of load too quickly or too slowly because of the body design, and other factors. Failure to consider any relevant factors may result in a body that does not properly shed the load.
Further reducing the angle of the floor “kick” (that is, less than the 5 degrees of a 12/17 body) typically results in a body that does not have adequate carrying capacity, as the load may slide off under transport, thus resorting back to a body that requires a tail gate, with the consequent construction and maintenance costs and risks of damage to the tail gate and loading buckets when loading the body. Also, if the floor angle is minimized, near the end of the payload dump cycle significantly more material ends up near the back of the floor. That material can suddenly slide and shed, causing the center of mass/gravity of the load to suddenly shift aft, leading to the risks of the cab suddenly rising (and falling when the material precipitously sheds) and sudden tension (and compression when the material sheds) on the lift cylinders. Therefore, further reductions in the angle of the body floor have typically not been successful.
Because combination bodies present significant possible advantages in haul truck fleet management, design of such a combination body can be greatly complicated by the real world requirements of length, center of load mass, and other such factors. However, a combination body design that might be readily adapted to be employed on different manufacturer's chassis would likely present significant advantages. Furthermore, a combination body having a significantly reduced tendency for the center of mass of the load to shift too far to the rear when dumping would likely be highly advantageous. Indeed, many of these advantages would also be quite useful on a high volume body carrying a high density material as well as various other types of bodies.